Recent advances in a communication technology, such as the availability of high-speed switching semiconductor devices, have enabled the use of transmitting and receiving a sequence of very short-duration radio frequency (RF) pulses, where the pulse duration is typically less than a nanosecond. Such a communication technology is sometimes referred to as “impulse radio (IR).”
Using very short-duration RF pulses, the IR technology can provide signal transmission over an ultra wide frequency bandwidth. In the IR technology, transmission is performed using an extended wide frequency bandwidth. Therefore, the average power spectral densities, although depending on the pulse repetition frequency and pulse amplitude levels, are in a very low region, for example, of 10−11 watts per Hertz. This low power emission can minimize interference with other wired or wireless systems operating in the same frequency band. In addition, a wide bandwidth in IR technology provides many advantageous characteristics for short range communications, such as a very large communication capacity at short distances.
However, the same characteristics that give prominence to the IR technology also lead to design challenges. One challenge is clock acquisition. Due to very short pulse duration in the IR technology, the acquisition process should be fast enough to support the IR data communication in a short time to a degree not considerably reducing the communication capacity. Furthermore, since the IR communication uses various modulation schemes, the clock acquisition also should provide accurate clock acquisition for various modulation schemes.
The clock acquisition for the IR technology can be classified into series search, parallel search and hybrid search, based on the acquisition method.
A sliding window series search can require searching for the signal through a number of dwell intervals in time (for example, Patent Document 1 and Non-Patent Document 1). In sliding window series search, a received signal and template signal are correlated and integrated at a receiver, and the baseband output is compared with a predetermined threshold. If the baseband output is larger than or equal to the threshold, the clock acquisition is completed. If the baseband output is less than the threshold, the template signal will be shifted or delayed by a predetermined time slot. The shifted or delayed template signal will be used in the receiver to repeat the operation of correlation, integration and threshold comparison. The operation is repeated until either the baseband output is larger than or equal to the threshold or all the shifted or delayed template signals are tested.
Non-Patent Document 2 explains several modifications on the sliding window series search, for example, random permutation search and bit reversal search. According to the simulation results in Non-Patent Document 2, the bit reversal search has a faster acquisition time than the sliding window search. However, the basic structure for series search mainly includes a single correlator, an adder and a threshold comparator.
A parallel search uses a receiver which adopts a configuration with a plurality of branches. In parallel search, each branch of the receiver has a correlator and an adder to do the same operation as series search. All branches carry out the operation simultaneously, and the maximum baseband output among these branches is compared with a predetermined threshold for signal acquisition.
Hybrid search works as a combination of series search and parallel search (for example, Non-Patent Document 3).    Patent Document 1: Japanese Patent Application Laid-Open No. HEI 6-74237    Non-Patent Document 1: “Rapid acquisition for ultra-wideband localizers”, Robert Fleming, Cherie Kushner, Gary Roberts, Uday Nandiwada, IEEE UWBST2002, May 2002    Non-Patent Document 2: “Rapid acquisition of ultra-wideband signals in the dense multipath channel”, Eric A. Homier, Robert A. Scholtz, IEEE UWBST2002, May 2002    Non-Patent Document 3: “Hybrid fixed-dwell-time search techniques for rapid acquisition of ultra-wideband signals”, Eric A. Homier, Robert A. Scholtz, International Workshop on UWB Systems, June 2003 Non-Patent Document 4: “Ultrawide bandwidth Time-Hopping Spread Spectrum Impulse Radio for wireless multiple-access communications”, M. Z. Win, R. A. Scholtz, IEEE Transaction on Communications, vol. 48, pp. 679-691, April 2000